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Global warming stretches subtropical boundaries

The atmosphere is warming faster in subtropical areas – around 30° north and south of the equator – than anywhere else in the world, according to a study of more than 25 years of satellite data. And the process appears to be driven by climate change.

Researchers analysed the tropical air circulation pattern – which brings rain to the equator and dries the subtropics – and found that this climate belt has expanded by about 1° of latitude, or about 70 miles, towards both the north and south poles over the past 27 years.

This in turn means that the north and south jet streams – the rivers of fast-moving air that form the boundaries between warm tropical air and cold polar air – have also moved correspondingly closer to each pole.

This worrying new feature of global warming could cause deserts to expand into heavily populated urban areas and see a desert-like climate encroach upon Europe, the researchers say. And the change could expand arid zones further from the equator to mid-latitude regions, possibly cutting winter rain and snowfall in the Alps, the Mediterranean and south-western Australia.

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Sahara spread

“The jet streams mark the edge of the tropics so, if they are moving poleward, that means the tropics are getting wider,” says John Wallace, who took part in the study at the University of Washington in Seattle, US.

“If they move another 2° to 3° poleward in this century, very dry areas such as the Sahara desert could expand further towards the pole, perhaps by a few hundred miles,” he adds.

The researchers used evidence from satellite measurements of temperatures in the lower atmosphere from the start of 1979, through to the end of 2005. It showed that the lower atmosphere warmed most over that period, at latitudes of 15° to 45° in both hemispheres.

The warming raises air pressure, affecting air circulation between those latitudes and the equator, explains team member Thomas Reichler, at the University of Utah, US.

Circulation loop

The tropical circulation pattern, called a Hadley cell, starts with hot air that rises at the equator until it reaches the stratosphere (upper atmosphere) where it cools and spreads toward both poles.

When it has spread into subtropical latitudes – around 30° degrees latitude from the equator – this now cooler air drops down towards the surface. The cool air then flows back to the equator, closing a circulation loop.

The Hadley circulation produces both tropical downpours and subtropical drought. Moisture condenses out of air rising near the equator, producing heavy rains. But the air that reaches the stratosphere is quite dry, and creates arid zones, such as the Sahara.

Desertification

It is this dry air that is now pushing further poleward. It is moving southwards from south Australia, South Africa and southern South America; and northwards from the southern US, south China and north Africa – threatening increased desertification.

“The main impact one would worry about is the effect on precipitation,” says Isaac Held of the NOAA Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey, who is not part of the team.

As the descending of dry air, or “downwelling”, from the upper atmosphere moves toward the poles, the droughts already experienced in the Mediterranean and south-western Australia could get worse.

The changes observed so far match those predicted by computer models for greenhouse warming of 0.5°C, Reichler told New Scientist. He suspects global warming is behind the changes, but cautions that the mechanisms pushing the expansion of the tropics are not yet understood.